Semiconductor integrated circuits (ICs) use metallic interconnects to connect individual devices on a chip. A major challenge to the continued scaling of IC technologies is the electromigration failure of the metallic interconnects. Electromigration refers to the phenomenon of electric current induced metal self-diffusion. Stated briefly, electromigration is the transport of material in a conductor arising from the momentum exchange between the electron currents (the “electron wind” force). Electromigration induced material depletion will lead to the development of tensile stress, while accumulation leads to the development of compressive stress at blocking boundaries. A backflow flux originates from the stress gradient and counters the electromigration flux. If the stress exceeds a critical value required for void nucleation, the line will fail. It is important to assess IC metallization reliability because of the high current densities that circuit interconnects carry. For example, thin film IC interconnects carry relatively high current densities in the range of 105 to 106 A/cm2, which leads to a large flux of atoms in the direction of electron flow. Therefore, there is a need to design and/or manufacture ICs that can withstand the electromigration impact for the target product lifetime at the target current density.
In one approach, dummy vias (or via plugs) are added to a conductor. A dummy via is non-functional—it does not form part of a signal line. A dummy via is also passive—it is not biased to any voltage. A dummy via is connected to the conductor at one end, and is left floating at the other end. The dummy via becomes a passive atomic reservoir for the conductor. Such approach generally has minor impact on electromigration because the top surface of the conductor is not the dominant electromigration diffusion path. Studies have shown that vias are places of atomic flux divergence, making them a primary electromigration reliability concern. In another approach, dummy lines are added to a conductor by extending the width of the conductor at various places. Such dummy lines become passive atomic reservoir for the conductor. Such approach has its own drawback. When the conductor's current changes direction, a previous passive atomic reservoir may become a passive atomic sink, which worsens the lifetime of electromigration. Accordingly, improvements in these areas are needed.